With two-factor authentication the code is sent to MY PHONE. If my phone is stolen, how do I use 'find my phone' since they will send the code to MY PHONE- which was stolen??? If my car is stolen with my phone in it, how do i find my phone?

The code will be sent to all trusted devices. If you have another iOs device or Mac you can get the code from there.

Beyond that, you can add trusted phone numbers you can have the code sent to, via voice call or SMS. They can be numbers from non apple devices (android) and even landlines. The trusted phone number should not be your iPhone's own number, but a different one from your home or from a friend or relative.

Link-> Two-factor authentication for Apple ID - Apple Support

To find your iPhone, you can log in to iCloud.com on any device or computer with your Apple ID and use the Find My app there to track your phone. Link-> Locate a device in Find My iPhone on iCloud.com - Apple Support

uselesstwofactorauthentication wrote:

With two-factor authentication the code is sent to MY PHONE. If my phone is stolen, how do I use 'find my phone' since when I try to sign in (using someone else's phone),

You don't need the code to access Find My iPhone.

Use any web browser on any platform. Go to https://www.icloud.com/find/ . As long as you include the /find in the URL, then no 2FA code is required to login.

You should also have a backup trusted phone number if you only have a single Apple trusted device. A trusted number can receive codes as SMS text or automated voice message. In the USA if you have a Google voice number, that will work fine as a backup trusted number.